Corrosion acid affected metals

Electrochemical studies have shown that inhibitors in acid solutions may affect the corrosion reactions of metals in the following main ways. 

The austenitic stainless steels that are not stabilized or that are not of the extra-low-carbon types, when heated in the temperature range of 450 to 843°C (850 to 1,550°F), have chromium-rich compounds (chromium carbides) precipitated in the grain boundaries. This causes grain-boundary impoverishment of chromium and makes the affected metal susceptible to intergranular corrosion in many environments. Hot nitric acid is one environment which causes severe... 

The acidic polysaccharides typically associated with biofilms can exhibit corrosive behavior toward metal surfaces (5-7). Bacteria within the viscous matrix grow, divide, and produce metabolites. The metal surface on which the bacteria grow is ultimately affected by these processes, of which material deterioration and corrosion are possible consequences. 

It is the intent of this paper to review some of the methodology and results of the NBS-BCL studies, and their relevance to the economic effects of acid rain. Since the effect of acid rain on materials would appear to be a form of corrosion (although affecting all materials, not only metals), the NBS corrosion study (1,2) will be the principal one discussed. 

One of the reasons for local corrosion at the metal-polymer interface is sorption of electrolytes by polymers and permeability of the polymer barrier towards electrolytes. Sorption of electrolytes (acid solutions, bases and salts) leads to essential variation in the service characteristics of the protecting polymer coatings and anticorrosion packaging films under mechanical loads. These variations under mechanical loads, especially in seals and friction joints, are much deeper and can affect mechanisms of contact interactions. 

The corrosion processes affecting lead when polarized within the Pb02 potential region (Eh > +0.95 V vs Hg/Hg2S04 electrode) are of major practical interest. Lead electrodes operate in this potential interval when used as positive electrodes in a lead—acid storage battery or as nonsoluble anodes during the refining of some metals. 

A corrosion inhibitor is a substance that efiectively reduces or prevents the corrosion of exposed metal in a corrosive environment. Inhibitors in small concentrations are added to cooling water, acid, and steam to maintain an inhibiting surfece film. Different mechanisms inhibit corrosion. This chapter emphasizes corrosion inhibitors that affect anodic or... 

Corrosion can affect an entire surface of a metal or just local spots. Uniform corrosion of the complete surface usually only happens in acidic conditions [2]. This generally results in overall thinning and causes no major damage. On the other hand, a very detrimental form of corrosion is pitting. This type is found at a single location on the surface and creates a pit or cavity which is difficult to prevent and often hard to detect. It can result in structural failure (example a cracked pipe). 

Nonstandard operations hint at the need for supplementary corrosion testing. For example, chemical cleaning operations are sometimes more damaging towards the materials of construction than the process itself. Hydrochloric acid is often used for chemical cleaning because of its low cost and rapid dissolution of metal oxides. Hydrochloric acid affects specific materials of construction, such as stainless steels, by means other than general corrosion. 

Pitting is also promoted by low pH. Thus, acidic deposits contribute to attack on stainless steels. Amphoteric alloys such as aluminum are harmed by both acidic and alkaline deposits (Fig. 4.4). Other passive metals (those that form protective corrosion product layers spontaneously) are similarly affected. 

Sulfur dioxide emissions may affect building stone and ferrous and nonferrous metals. Sulfurous acid, formed from the reaction of sulfur dioxide with moisture, accelerates the corrosion of iron, steel, and zinc. Sulfur oxides react with copper to produce the green patina of copper sulfate on the surface of the copper. Acids in the form of gases, aerosols, or precipitation may chemically erode building materials such as marble, limestone, and dolomite. Of particular concern is the chemical erosion of historical monuments and works of art. Sulfurous and sulfuric acids formed from sulfur dioxide and sulfur trioxide when they react with moisture may also damage paper and leather. 

The metal lost from the inside of pumps, reaction vessels, pipework, etc. usually contaminates the product. The implications of this depend upon the product. Ppb levels of iron can discolor white plastics, though at this level the effect is purely cosmetic. Ppm levels of iron and other metals affect the taste of beer. Products sold to compositional requirements (such as reagent-grade acids) can be spoiled by metal pick-up. Pharmaceutical products for human use are often white tablets or powders and are easily discolored by slight contamination by corrosion products. 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

Some of the worst corrosive effects in soft waters are attributed to a rather wide group of organic acids abstracted from peat and mosses, sometimes called peaty acids. Such waters have low pH values and are often discoloured. They affect ferrous metals appreciably and also attack lead and... 

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